In radiometric level measurement or radiometric density measurement, ionizing radiation is passed through the container (tank or silo) in which the filling material is stored. In general there are two known kinds of built-on accessories: Either the radiation originates at a point-like transmitting unit at the upper edge of the container and is detected by a rod-shaped receiving unit (scintillator) which extends over the entire fill level on the opposite side of the container, or the transmitting unit is rod-shaped and the receiving unit is embodied as point-like. In the latter configuration, the receiving unit is preferably located in the upper region of the container.
The receiving unit is made of either plastic or a crystal. In either case, the gamma radiation arriving from the container or passing through the container is at least partly absorbed in the receiving unit. The absorbed radiation is partly output again in the form of UV light. Since in plastic the transmission of UV light is very slight, a wavelength shifter is typically incorporated into the plastic as well. This wavelength shifter converts the UV light into visible light (as a rule, blue or green light). The converted light can subsequently be converted into electrical signals, for instance by a photomultiplier. The electrical signals are then evaluated in an electronic circuit. For the sake of evaluation, in the normal situation, the normal of light pulses is counted. Another possible way of evaluation is to examine the amplitude spectrum, that is, the number of pulses, sorted by their amplitude. In both cases, it is always the entire radiation that has passed through the container and the filling material located in the container that is assessed. Depending on the level or density of the filling material, the proportion of the absorbed radiation is accordingly more or less high.
The known radiometric level measuring devices have the following disadvantages:                The outcome of measurement is affected by the temperature dependency and by the sensitivity of the detector.        If relatively long plastic rods are used as a receiving unit, then a large proportion of the light is absorbed; in that case, amplitude evaluation is of little use.        Extraneous sources that are not part of the measurement setup adulterate the outcome of measurement.        Different wall thicknesses of the container must be compensated for by complicated calculations.        Since the intensity of the radiation decreases with the square of the distance between the transmitting unit and the receiving unit, a linearization of the outcome of measurement must be performed.        Another disadvantage is that with increasing rod length, the angle of incidence of the radiation at the container wall becomes greater. This increases the travel length in the container wall and thus the absorption. Since the absorption function is exponential, this effect increases disproportionately greatly as the wall thickness increases.        
Given the above relationships, the measurement accuracy of the system decreases with increasing distance from the transmitting unit, which means that the measurement accuracy is higher in the upper region of the container than in the lower region. The ratio of the fill level to the container diameter is limited to approximately 1/1. The consequence of this is that upon measurement over an extended range of levels, a plurality of transmitting units are needed. In approximate terms, it can be said that because of the absorption of the light in the plastic rod, the measurement range for a single detector unit is limited to approximately two meters.